The present invention relates to a knee prosthesis and more particularly to a mobile bearing knee providing posterior stabilization of the anterior-posterior translation of the femoral component relative to the tibial component.
Flexion and extension of the normal human knee involves complex movements of the femur, the tibia and the patella. During flexion (i.e., when the knee is bent), the distal end of the femur and the proximal end of the tibia roll and glide relative to each other, with the center of rotation of the joint moving posteriorly over the condyles of the femur. This complex movement is typically referred to as rollback. During extension (i.e., when the leg is straightened), the tibia and femur follow a reverse path. Simultaneous with the movements of the tibia and femur, the patella moves over the surface of the femoral condyles, while remaining a constant distance from the tubercle of the tibia.
Damage or disease can deteriorate the bones, articular cartilage and ligaments of the knee, which can ultimately affect the ability of the natural knee to function properly. To address these conditions, prosthetic knees have been developed that are mounted to prepared ends of the femur and tibia. Among the many knee prostheses, a mobile bearing knee simulates the condylar and bearing surfaces of the knee to emulate the natural movement of the knee during flexion and extension. The tibial component is configured to permit rotation about the axis of the tibia to accurately replicate the effects of differential rollback in the transverse plane.
In one type of mobile bearing knee, the tibial component includes an upward projecting spine that translates within an intercondylar notch formed in the femoral component, The spine can contact cam surfaces at the anterior and posterior ends of the notch to limit the relative anterior-posterior movement between the two bones. The spine also operates to provide varus-valgus stability of the joint and to resist dislocation or subluxation at high angles of flexion. An exemplary mobile bearing knee is disclosed in U.S. Pat. No. 6,443,991, the disclosure of which is incorporated herein by reference. Other exemplary mobile bearing knees are embodied in the LCS™ System and the PFC Sigma RP™ knee system marketed by Depuy Orthopaedics, Inc., of Warsaw, Ind.
While mobile bearing knees are thought to most accurately mimic the natural movement of the intact knee, the design of knee systems requires the introduction of features to maintain the stability of the artificial joint. Thus, modern knee systems provide additional stability to posterior stabilized devices to prevent hyperextension. The articulating and rotating components of the knee system must do so smoothly and accurately. At the same time, the natural knee permits a certain amount of movement and pivoting in the transverse and coronal planes that should be approximated in the prosthetic knee system. The development of knee systems has attempted to harmonize the need for preserving a full range of motion with the need for maintaining the strength of the joint.